With the rapidly growing trend toward cordless and more portable consumer electronic products, there is an increasing demand for batteries that are small and lightweight but have high energy density as the power source for such electronic products. From this viewpoint, non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are commonly used in such appliances as notebook computers, mobile phones and audiovisual equipment as batteries having high voltage and high energy density.
Because of the use in such appliances, these non-aqueous electrolyte secondary batteries are required to have good discharge characteristics while maintaining high capacity. In order to satisfy the requirement, it is essential to improve permeability of an electrolyte into a positive electrode.
The permeability of the electrolyte into the positive electrode can be improved, for example, by a process of adding a proper amount of fluorocarbon resin having a good affinity for the electrolyte to a positive electrode material layer as a binder while ensuring sufficient porosity of the positive electrode material layer. The expression “porosity” as used herein refers to the volume ratio of pores of the positive electrode material layer.
Fluorocarbon resins conventionally used include polyvinylidene fluoride (hereinafter referred to as PVDF) and polytetrafluoroethylene (hereinafter referred to as PTFE). Among them, PVDF is versatile as the binder. One of the reasons for this is that PVDF, which has a high affinity for a non-aqueous electrolyte, facilitates permeation of the electrolyte into the positive electrode material layer.
For example, a fluorocarbon resin dissolved in a solvent such as N-methyl-2-pyrrolidone (hereinafter referred to as NMP) is added as the binder, together with carbon black such as acetylene black and Ketjen black serving as a conductive agent, to a lithium-containing transition metal oxide serving as a positive electrode active material, and the resultant mixture is kneaded to prepare a positive electrode material paste. The positive electrode material paste is applied onto a positive electrode core member such as an aluminum foil and dried, and the resultant member is rolled and cut into predetermined dimensions. This is how a positive electrode is conventionally produced.
The affinity between a solid and a liquid is generally evaluated by dropping the liquid on the surface of the solid and measuring the contact angle formed therebetween. When the affinity between the solid and the liquid is high, i.e., when there is a small difference in surface free energy between the two, the contact angle becomes small. For example, the contact angle between a PVDF film and a non-aqueous electrolyte having a common composition composed of a mixed solvent of ethylene carbonate and diethyl carbonate and LiPF6 dissolved therein is as low as about 15°.
The above-described conventional method of positive electrode production, however, has a problem that the conductive agent tends to form agglomerated particles including the fluorocarbon resin. Although the cause of this problem is uncertain yet, the reason may be as follows. The conductive agent generally has a low affinity for polar solvents such as NMP, water and solvents to be used for non-aqueous electrolytes, and the fluorocarbon resin dissolved in such a polar solvent is therefore deposited in sequence on the surface of the conductive agent to promote formation of the agglomerated particles. This causes a shortage of the binder in the vicinity of the positive electrode active material particles to hinder smooth permeation of the electrolyte to be ensured by the presence of the binder, so that electrode reactions are hampered, resulting in lowered discharge characteristics. Also, with the formation of the agglomerated particles, because of the shortage of the binding effects of the binder, there arises another problem that the positive electrode material layer separates from the positive electrode core member.
On the other hand, when an excessive amount of binder is added in order to ensure that the electrode sheet is bound firmly, it is difficult to heighten the battery capacity. That is, an addition of a large amount of conductive agent and binder to the positive electrode, in consideration of the agglomeration of the conductive agent, makes the active material density low. The expression “active material density” as used herein refers to the density of the active material (g/ml) obtained by dividing the weight of the positive electrode active material contained in the positive electrode material layer by the volume of the positive electrode material layer. Resultantly, it becomes difficult to realize a non-aqueous electrolyte secondary battery having a higher capacity.
The present invention aims to provide a non-aqueous electrolyte secondary battery having a high capacity and excellent characteristics in terms of charge/discharge and cycle life. The present invention improves the way the conductive agent and binder are used and controls the amounts of the conductive agent and binder to attain high active material density of the positive electrode.